Diffractive stack pickup head for optical disk drives and method to fabricate the pickup head

ABSTRACT

The invention provides a pickup head used in an optical data storage system. The pickup head contains a diffractive optical element (DOE) stack on a semiconductor substrate. The DOE stack includes multiple diffractive lenses for providing several diffractive surfaces and a middle layer serving as a beamsplitter and servo-generating element for a light reflected from an optical storage medium. The middle layer is sandwiched by the diffractive lenses that are located on both outer parts of the DOE stack. The semiconductor substrate includes at least a laser source and several photodetectors. The middle layer serving its function preferably includes a polarization-selective DOE and a quarter-wave retarder oriented to rotate a polarization state of the laser source, so that only the light reflected from the optical storage medium is diffracted by the polarization-selective DOE.

BACKGROUND OF THE INVENTION

[0001] 1. Field of Invention

[0002] The present invention relates to an optical electronictechnology. More particularly, the present invention relates to adiffractive stack pickup head for an optical disk drive and a method tofabricate the diffractive stack pickup head

[0003] 2. Description of Related Art

[0004] Pickup heads are an opto-electronic means by which a signal iswritten to or read from an optical disk. FIG. 1 is a system drawing,schematically illustrating a conventional optical disk pickup system. InFIG. 1, the generic optical disk pickup system includes a laser diodelight source 20, a beamsplitter 22, and at least one lens 24 forprecisely focusing the light from the laser diode 20 onto an informationbearing surface of the optical disk 26. The system further includes amulti-element photodetector 28 for receiving light that is reflectedfrom the disk 26, and a servo-generating element 30, such as thecylindrical lens, which is used to alter the reflected beam from thedisk 26 into some form whereby the state of focus and tracking can beeasily extracted from the signals generated by the light incident on themulti-element photodetector 28. This conventional system can be referredto A. B. Marchant, Optical recording, A Technical Overview,Addison-Wesley Publishing, Menlo Park Calif., 1990, p.197. The generalmounting arrangement for these optical elements into a pickup head unitis shown in FIG. 2. In FIG. 2, each component is mounted separately, oneat a time, into a pickup head housing 32. The size of the housing 32 isdirectly related to the image distance of the focusing lens(es) 24,i.e., the distance from the laser diode 20 to the focused image of thelaser diode 20 on the disk 26 of FIG. 1. With current technology ofrefractive molded aspheric lenses, this distance is typically in therange of 25-40 mm, and the size of the housing is roughly of the sameorder. In some designs, the servo-generating element can be an integralpart of the beamsplitter 22 and sometimes is implemented using adiffractive or holographic optical element (DOE). FIG. 3 is a pickupsystem drawing, schematically illustrating an alternate generic pickupdesign where the beamsplitter and servo-generating elements are combinedinto a single DOE and the complete optical system excluding theobjective lens is packaged in a small module. In FIG. 3, the design ofthe pickup system 38 has the advantage that most of the opto-electroniccomponents 40, except the focusing lens/actuator are assembled in aminiature module. The components 40 are also assembled one at a time toform the miniature module. To complete the pickup system, the components40 are mounted in a pickup housing similar to the one shown in FIG. 2.However, the general size of the pickup system 38 is still determined bythe imaging distance of the focusing lens. In other words, theholographic module approach has certain advantages but it does notsignificantly change the size or weight of the complete pickup system.

SUMMARY OF THE INVENTION

[0005] The invention provides an optical disk pickup head on the currentthe state-of-the-art technology having an objective that the inventionallows many pickups to be made simultaneously in parallel, therebysignificantly reducing manufacture cost.

[0006] The invention also includes another objective that the entirepickup, including the focusing lens, can be made in a very smalldimension, such as a few millimeters on a side. This allows multiplepickups to be utilized in a single drive.

[0007] As embodied and broadly described herein, the invention providesa pickup head used in an optical data storage system. The pickup headincludes a diffractive optical element (DOE) stack on a semiconductorsubstrate. The DOE stack includes multiple diffractive lenses forproviding several diffractive surfaces and a middle layer serving as abeamsplitter and servo-generating element for a light reflected from anoptical storage medium. The middle layer is sandwiched by thediffractive lenses that are located on both outer parts of the DOEstack. The semiconductor substrate includes at least a laser source andseveral photodetectors.

[0008] The middle layer serving its function preferably includes apolarization-selective DOE and a quarter-wave retarder oriented torotate the polarization state of the laser source, so that only thelight reflected from the optical storage medium is diffracted by thepolarization-selective DOE.

[0009] It is to be understood that both the foregoing generaldescription and the following detailed description are exemplary, andare intended to provide further explanation of the invention as claimed.

BRIEF DESCRIPTION OF THE DRAWINGS

[0010] The accompanying drawings are included to provide a furtherunderstanding of the invention, and are incorporated in and constitute apart of this specification. The drawings illustrate embodiments of theinvention and, together with the description, serve to explain theprinciples of the invention. In the drawings,

[0011]FIG. 1 is a system drawing, schematically illustrating aconventional optical disk pickup system;

[0012]FIG. 2 is a dissected perspective view, schematically illustratinga conventional mounting structure of optoelectronic components into apickup head unit;

[0013]FIG. 3 is a pickup system drawing, schematically illustrating analternate generic pickup design where the beamsplitter andservo-generating elements are combined into a single DOE and thecomplete optical system excluding the objective lens is packaged in asmall module;

[0014]FIG. 4 is a cross-sectional view, schematically illustrating apickup head including the DOE stack formed on a semiconductor substrate,according to the preferred embodiment of the present invention;

[0015]FIG. 5 is a drawing, schematically illustrating a concept of afabrication method for the DOE stack according to the preferredembodiment of the invention, in which a 2-dimensional array of elementsis first fabricated on each wafer layer, and then the wafer layers arealigned and bonded together where the wafers are cut into strips of1-dimensional arrays of DOE stack elements or into individual DOE stackelements;

[0016]FIG. 6 is a top view, schematically illustrating the structure ofthe pickup head on a the semiconductor substrate, according to thepreferred embodiment of the invention;

[0017]FIG. 7 is a drawing, schematically illustrating a method formounting the strips of 1-dimensional arrays of DOE stack elements ontothe array of laser diode/multi-element photodetector systems that hasbeen fabricated on the semiconductor substrate wafer, according to thepreferred embodiment of the invention;

[0018]FIG. 8 is a drawing, schematically illustrating a pickup systemusing the DOE stack according to a first preferred embodiment of theinvention, in which light path through the DOE stack is folded byseveral reflective DOE surfaces and the DOE stack contains bothreflective and transmissive DOE surfaces;

[0019]FIG. 9 is a drawing, schematically illustrating a structure forthe beamsplitter/servo-generating DOE and the multi-elementphotodetectors, according to the first preferred embodiment of theinvention;

[0020]FIG. 10 is a drawing, schematically illustrating the DOE for thebeamsplitter and the servo-signal generating element, according to thefirst preferred embodiment of the invention; and

[0021]FIG. 11 is a drawing, schematically illustrating a DOE stack,according to a second preferred embodiment of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0022] The invention utilizes a stack of diffractive optical elements(DOE) in contact with a semiconductor substrate to build a pickup head.The basic structure is shown in FIG. 4. FIG. 4 is a cross-sectionalview, schematically illustrating a pickup head including the DOE stackformed on a semiconductor substrate, according to the preferredembodiment of the present invention. In FIG. 4, a DOE stack 44 islocated on a semiconductor substrate 46. The DOE stack 44, for example,includes at least one diffractive lens surface 44 a, such as two, on thetop of the DOE stack 44, and includes at least one diffractive lenssurface 44 b on the bottom of the DOE stack 44, where the at least onediffractive lens surface 44 b preferably shown in two. The diffractivelenses 44 a, 44 b provides several diffractive lens surfaces. The DOEstack 44 also includes a quarter-wavelength retarder 44 c and apolarization-selective DOE 44 d performing a function of beamsplitter &servo-signal generation. The quarter-wavelength retarder 44 c and apolarization-selective DOE 44 d are sandwiched by the diffractive lenses44 a, 44 b, and the quarter-wavelength retarder 44 c is preferablylocated on the polarization-selective DOE 44 d. The DOE stack 44 canalso perform a focusing function. Moreover, the semiconductor substrate46 usually includes at least a laser diode and several photodetectors,for example, like those shown in FIG. 2.

[0023] During manufacturing, each layer in the DOE stack 44 is treatedas wafer-like, and is patterned by standard semiconductor processingtechnologies, such as processes of photolithography and etching. Thepattern of each layer includes, for example, a regular array of aparticular shape that is required for each individual layer according tothe pattern design. In other words, many identical copies of thediffractive surface pattern required by the design for a given layer ofthe DOE stack are fabricated simultaneously on the wafer. FIG. 5 is adrawing, schematically illustrating a concept of a fabrication methodfor the DOE stack according to the preferred embodiment of theinvention, in which a 2-dimensional array of elements is firstfabricated on each wafer layer, and then the wafer layers are alignedand bonded together where the wafers are cut into strips of1-dimensional arrays of DOE stack elements or into individual DOE stackelements. In FIG. 5, as each layer of the DOE stack 44 is formed withthe desired pattern, each layer is stacked up to form a preliminary DOEstack 44′ with two-dimensional pattern. The preliminary DOE stack 44′ isthen, for example, cut into several strips. Each strip is a set of theDOE stack 44 of FIG. 4. The layers on the outsides of the DOE stack 44are the diffractive lens surfaces 44 a, 44 b of FIG. 4 and are composedof, for example, glass, plastic, or other similar material.

[0024] The diffractive lens surfaces 44 a, 44 b can diffract light forany and all polarization states. The focusing power, which images thelaser diode onto the disk information surface with a high numericalaperture (NA) convergence of the light on the disk side, is distributedbetween three or four diffractive lens surfaces 44 a, 44 b of the DOEstack. In theory, the focusing power could be totally realized with onlyone or two DOE's. The requirement of high-NA, such as NA=0.6 for a DVDpickup head, leads to too fine a pitch for the grooves of the DOE, whichmakes them difficult or impossible to manufacture and adversely affectsthe diffraction efficiency of the DOE stack 44. By distributing thefocusing power between three ore four surfaces, uniformly highdiffraction efficiency can be achieved and the minimum groove spacing isincreased making fabrication feasible. Furthermore, it is well knownthat DOE's are highly dispersive. By using multiple diffractive surfacesthat together form the focusing lens, one can design these surfaces witha balance of positive and negative powers to achieve the desired amountof achromaticity. This cannot be achieved by only using a singlediffractive surface.

[0025] The sandwiched layers 44 c and 44 d of the DOE stack 44 of FIG. 4also include another DOE surface. This surface implements the functionsof beamsplitter and servo-generating element. The DOE pattern and themulti-element photodetector pattern, as mentioned in FIG. 5, aredesigned such that one can determine the information recorded on thedisk as well as the state of focus and tracking of the light spot on thedisk from the electrical signals. The electric signals are generated bythe light incident on the photodetectors. The servo-generating andbeamsplitting function can be implemented using either one or twolayers. When implemented with just one layer, this single layer replacesthe two layers 44 c and 44 d shown in FIG. 4. This single layer is madefrom an isotropic material and diffracts light of any polarizationstate. The forward-going light (i.e. towards the disk) that isdiffracted by this layer is not used. The return-path light, which isreturning due to reflection from the disk, is diffracted onto thephotodetectors. Alternatively, the servo-generating/beamsplitterfunctions may be implemented using two layers 44 c and 44 d as shown inFIG. 4. These two layers are either made from birefringent materials orby other means fabricated such that their influence on an optical beampassing through them depends on the polarization of the incident beam.The layer nearest the laser diode, 44 d, has a polarization-selectiveDOE fabricated on one surface. The laser emits linearly polarized light.As it propagates from the laser towards the disk, thepolarization-selective DOE is designed, and its birefringence axis isoriented, such that no diffraction occurs. The otherpolarization-dependent layer, 44 c, i.e., the one nearer the disk, isfabricated as a quarter-wave retarder and is oriented such that its axisis at 45 degrees to the direction of polarization emitted by the laser.Passing through this quarter-wave retarder layer 44 c twice, once as itpropagates from the laser to the disk and a second time as the lightreflected from the disk passes back through the pickup head, thepolarization direction of the light is rotated by 90 degrees. As aresult, when the light reflected from the disk reaches thepolarization-selective DOE surface 44 d, it is now of the correctpolarization orientation to be diffracted with high efficiency by thissurface onto the photodetectors.

[0026] The laser diode and the multi-element photodetectors arefabricated or mounted on the semiconductor substrate 46. The basiclayout of the components on the substrate 46 is shown in FIG. 4 andfurther shown in FIG. 6. FIG. 6 is a top view, schematicallyillustrating the structure of the pickup head on the semiconductorsubstrate, according to the preferred embodiment of the invention. Thestructure shown in FIG. 6 is one unit of a laser/photodetector systemelement 60. In FIG. 6, the semiconductor substrate 46 includes anindentation or well 50 where the laser diode 52 is located. The laserdiode 52 has a reflecting surface (mirror), also calledmirror/beamshaper 53, in front of the laser emission facet to turn thelaser beam by 90 degrees so that the laser beam exits perpendicular tothe plane of the semiconductor substrate 46. This reflecting surface mayalso have curvature or diffractive power so as to perform beamshaping onthe laser beam. In the production process, a semiconductor wafer 46 ispatterned with an array of the laser and multi-element photodetectorsystems 60 as described above. Each step of the fabrication process isperformed on all array elements in parallel, such as a typicalsemiconductor fabrication. In other words, several pickup elements arefabricated on each semiconductor wafer at the same time. Metal traces 54are also formed to allow several bonding pads 56 formed on the substrate46 to be electrically coupled to the photodetectors 48. It must be notedthat the bonding pads 56 should not be covered when the DOE stack 44 ismounted on the semiconductor substrate 46.

[0027] As the DOE stack 44 is to be assembled on the semiconductorsubstrate 46, the optical components of the DOE stack 44 must beproperly aligned and bonded to the optoelectronic laser diode and thephotodetector system on the semiconductor substrate 46. There arevarious ways to assemble the DOE stack 44. The semiconductor substrate46 and the DOE stack can be each diced into individual pickup elementsand bonded one by one. Or the DOE stack 44 can be diced into individualelements, and then each is mounted on the semiconductor substrate 44 atthe desired place. FIG. 7 is a drawing, schematically illustrating amethod for mounting the strips of 1-dimensional arrays of DOE stackelements onto the array of laser diode/multi-element photodetectorsystems that has been fabricated on the semiconductor substrate wafer,according to the preferred embodiment of the invention. In FIG. 7,several of pickup elements are fabricated in parallel. The DOE stack 44is a strip cut from the preliminary DOE stack 44′ of FIG. 5. Each DOEstack 44 includes an one-dimensional array of DOE stack elements, whichhave the same spacing as the laser/photodetector system elements 60 onthe semiconductor substrate 46. The one-dimensional array strips of DOEstack 44 can be aligned row by row with respect to rows of thelaser/photodetector system elements 60, each of which has an opening toexpose the bonding pads for leading electric signals between the DOEstack array strips 44. After the mounting the DOE stack 44 on thesemiconductor substrate 46, the substrate 46 is diced into individualpickups.

[0028] The pickup fabricated by the above method of the invention canhave various applications in the optical pickup head. Severalembodiments for the pickup system using the DOE stack are, for example,shown in the following.

[0029] Embodiment 1:

[0030]FIG. 8 is a drawing, schematically illustrating a pickup systemusing a DOE stack according to a first preferred embodiment of theinvention, in which light path through the DOE stack is folded byseveral reflective DOE surfaces and the DOE stack contains bothreflective and transmissive DOE surfaces. In FIG. 8, a distinctive partof this embodiment is that reflective DOE surfaces 1 and 2 are used astwo of the DOE lens surfaces. The need for a high NA on the disk side ofthe lens and a working distance on the order of a millimeter essentiallydetermines the beam size at the lens. The comparatively low NA of thebeam emitted by the laser diode 52 requires a certain distance to expandto this size, longer than the lens to the disk image distance by afactor of the ratio of lens NA to the beam NA. In this embodiment 1, atechnique similar to a reflecting telescope, which folds the opticaldistance into a small space, is used. Light enters the diffractive stackthrough at least a transparent opening at the reflective DOE surface 2.It is incident on reflective DOE surface 1 that is designed to reflectthe beam and to expand its divergence. In this design, we use adiffractive surface to diverge the beam, which allows the surface toremain flat and, therefore, stackable. The beam is diverged at DOEsurface 1, thereby increasing the beam NA so that it takes less opticalpath length to reach the desired beam cross-sectional diameter. Thisalso, in turn, allows the pickup to be fabricated in a smallerdimension. The DOE surface 2 again reflects the beam back in thedirection of the disk 26. Depending on the particular design, it mayhave converging, diverging, or have no focusing power. This degree offreedom in the design can also be used to achromatize the diffractivestack lens. In summary, by folding the beam with reflection surfaces,light passes through the first layer of the diffractive stack, forexample, three times. This results in that the needed optical pathlength is folded into a smaller physical space. By increasing the beamNA at DOE surface 1 and perhaps surface 2, the required optical pathlength is also reduced.

[0031] As the light beam again passes by DOE surface 1, the reflectiveDOE blocks a portion of the light beam, but most portion of the lightbeam passes through the transparent region outside of the DOE reflectiveregion. The beam polarization and the orientation of thepolarization-selective DOE 44 d are designed so that the forward-goinglight, which propagates from the laser diode 52 to the disk 26, is notdiffracted but rather passes directly through the polarization-selectiveDOE 44 d. After transiting the quarter-wave retarder 44 c, the linearlypolarized light becomes a circularly polarized light. The diffractiveDOE surface 3 and 4 both focus the light so that the desired high-NAconverging beam is incident on the disk 26. The diffractive powers ofthe DOE surfaces 1-4 are designed together so as to minimize thephysical size of the pickup and to share the required total focusingpower. In this manner, the pickup can achieve a high diffractionefficiency, a reasonable minimum groove spacing on the individual DOEsurfaces, and the necessary degree of achromaticity.

[0032] After reflection from the disk 26, the light beam again travelsthrough the DOE surfaces 4 and 3, and the quarter-wave retarder 44 c.The net result of traversing the properly-oriented quarter-wave retarder44 c two times, is that the light emerges linearly polarized withpolarization direction rotated by 90 degrees relative to the lightemitted by laser diode 52. This is polarization direction for whichdiffraction by the polarization-selective DOE is maximized. Thepolarization-selective DOE 44 d, serving the role of a beamsplitter,diffracts the light beam towards the multi-element photodetectors 48(see FIG. 6) on the semiconductor substrate 46, accessable throughtransparent openings at the DOE surface 2. The polarization-selectiveDOE 44 d also play the role of a servo-generating element, which is usedfor conjunction with the multi-element photodetectors 48 to produce anelectronic signal that is used to determine whether or not the light isproperly focused on the disk 26 at its surface. The signal is alsocalled the focus error signal (FES). A tracking error signal (TES) isalso produced to determine whether or not the focused spot is properlyfollowing the track of information on the disk 26. In general, thepolarization-selective DOE 44 d and the photodetectors 48 can bedesigned to implement any of a number of well-known servo-generatingmethods, including but not limited to the astigmatic method, theknife-edge method, or the spot-size detection method for generating FESsignals. Moreover, the TES signals can be generated by, for example, apushpull (PP) and differential phase detection (DPD) methods. Theradio-frequency (RF) signal containing the information stored on thedisk 26 is the sum of the electrical signals from all of themulti-element photodetectors 48.

[0033] A preferred method to form a compatible servo-generating elementis described in the following. FIG. 9 is a drawing, schematicallyillustrating a structure between the beamsplitter/servo-generating DOEand the multi-element photodetectors, according to the first preferredembodiment of the invention. In FIG. 9, the optical beam can besubstantially centered on this polarization-selective DOE 44 d. It isdivided into four regions, each of which directs part of the light beamfrom the disk onto a different photodetector element 48. FIG. 10 is adrawing, schematically illustrating the DOE for the beamsplitter and theservo-signal generating element, according to the first preferredembodiment of the invention. The light may either go directly from thepolarization-selective DOE 44 d to the photodetectors 48 as shown in thelower drawing, or it may be reflected by the reflective DOE surfacesbefore it reaches the photodetectors as shown in the upper drawing.

[0034] Both FIG. 9 and FIG. 10 schematically illustrate one preferreddesign for the beamsplitter/servo-generating DOE 44 d and themulti-element photodetectors 48 that is compatible with the stack pickupdesigns of the invention. The X-axis corresponds to the tangential diskdirection and the Y-axis to the radial disk direction. In FIG. 9, theparts of the light beam reflected from the disk, falling on the DOE ABand the DOE CD, are diffracted onto the multi-element photodetectors ABand CD respectively. The DOE AB and the DOE CD in conjunction with themulti-element photodetectors AB and CD implement a method of focus errordetection. The focus error signal is given by:

FES=(SA−SB)−(SC−SD),

[0035] where SA, SB, SC, and SD indicate the electrical signalsgenerated by light falling on the photodetectors A, B, C, and Drespectively. The axis of the central strips of the multi-elementphotodetectors 48 AB and CD, such as the strips B and D, are at +/−45degrees relative to the Y-axis. In this manner, the optical spotsdiffracted by DOE AB and DOE CD remain centered on the central strips ofdetectors AB and CD independent of slight wavelength shifts of the laserlight that invariably occur with aging or temperature change. As aresult, the proper FES is maintained despite shifts in wavelength. Theparts of the beam reflected from the disk that fall on DOE E and DOE Fare diffracted onto photodetector elements E and F respectively. Theyare used to provide DPD and pushpull (PP) tracking error signals asgiven by the following expressions:

TES _(DPD)=phase(SA+SB+SE)−phase(SC+SD+SF),

TES _(PP)=(SA+SB+SC+SD)−(SE+SF),

[0036] where SE and SF indicate the electrical signals generated bylight falling on photodetectors E and F respectively. The RF signalcontaining the information stored on the disk is the sum of theelectrical signals from all of the multi-element photodetectors 48. Inthis embodiment 1, the photodetectors are on the opposite side of theoptical axis, (that is, the axis running from the laser diode to thefocused spot on the disk), from the quadrant elements of DOE AB and DOECD. The method of the invention makes use of the reflective anddiffractive functions of DOE surfaces 1 and 2 in the optical path of thelight diffracted by DOE AB and DOE CD of the polarization-selective DOE44 d to bring the light across the optical axis and onto thephotodetectors AB and CD. This is schematically shown in FIG. 10. InFIG. 10, the upper drawing shows the path of the light diffracted by DOEAB or DOE CD and the lower drawing shows the path of the lightdiffracted by DOE E or DOE F.

[0037] Embodiment 2.

[0038]FIG. 11 is a drawing, schematically illustrating a DOE stack,according to a second preferred embodiment of the invention. In FIG. 11,light travels through the DOE stack 44 in one direction from the laserdiode to the disk, and in the reverse direction from the disk to thephotodetectors. The embodiment 2 of FIG. 11 is basically similar to theembodiment 1 of FIG. 8, but the DOE stack 44 contains only transmissivesurfaces and no reflective surfaces. All DOE surfaces 1′, 2′, and 3′ arenow transmissive. Taking the optical axis from the laser to the focusedspot on the disk to be the Z-direction, the light travels completely inthe positive Z (+Z) direction from the laser to the disk and completelyin the negative Z (−Z) direction from the disk to the multi-elementphotodetectors. In this manner, three or more transmissive DOE surfacesare used, in which three DOE surfaces are shown in FIG. 11. Thetransmissive DOE surfaces are used to focus the light beam from thelaser diode onto the disk with high NA. The DOE surface 1′ expands thedivergence of the light beam from the laser diode so that the light beamcan achieve its desired size in a shorter optical path length. The DOEsurface 2′ and 3′ share the work of focusing the light beam so that thedesired high-NA converging beam is incident on the disk 26. Thediffractive power of the DOE surfaces 1′, 2′, and 3′ are designedtogether to minimize the physical size of the pickup and to share therequired focusing power such that a high diffraction efficiency,reasonable minimum grooving spacing, and the necessary degree ofachromaticity can be achieved.

[0039] As in embodiment 1, the polarization-selective DOE 44 d isoriented such that it does not diffract the linearly polarized lighttravelling from the laser diode to the disk 26. The quarter-waveretarder 44 c is oriented such that when the light beam passes throughthe quarter-wave retarder 44 c twice, once on the way towards the disk26 and once on the return trip after reflection from the disk, thelinear polarization direction of the laser light is rotated by 90degrees. This is the polarization state for which diffraction by thepolarization-selective DOE 44 d is maximized. The polarization-selectiveDOE 44 d, as in embodiment 1, serves the role of a beamsplitter,diffracting the light beam onto the multi-element photodetectors, andalso the role of servo-generating elements, altering the amplitude andphase of the diffracted light field in such as way that appropriate FESand TES signals can be extracted from the electrical signal generated bythe multi-element photodetectors. Alternatively, as in the previousembodiment, the polarization-selective DOE 44 d and the quarter-waveretarder 44 c can both be replaced by a singlenon-polarization-selective DOE that implements the servo-generating andbeamsplitting functions, albeit with less efficiency than the two-layermethod. Any of the standard servo approaches can be used including, butnot limited to, the particular method described above (FIG. 9) or theastigmatic, knife-edge, or spot-size detection methods for generatingFES, and the pushpull or DPD method for generating the TES can be usedin this design. The RF signal containing the information stored on thedisk is the sum of the electrical signals from all of the multi-elementphotodetectors.

[0040] Embodiment 3.

[0041] Embodiment 3 is similar to embodiment 1 except the edge-emittinglaser diode 52 and the turning mirror/beamshaper on the semiconductorsubstrate are replaced by a vertical cavity surface emitting laser(VCSEL) source. The VCSEL is a type of semiconductor laser that emitslight from the top flat laser chip surface rather than from its edge.Since it emits light form its top surface, the VSCEL can be mounted flatwithin the well in the semiconductor substrate and the light naturallypropagates towards the diffractive stack. No turning mirror is required.Furthermore, the light beam emitted by the VCSEL has a circularcross-section and no astigmatism. Therefore, the beamshaper is also notnecessary.

[0042] Embodiment 4.

[0043] The invention also provides the embodiment 4, which is similar toembodiment 2 but the edge-emitting laser diode and the turningmirror/beamshaper on the semiconductor substrate are replaced by a VCSELsource.

[0044] The invention provides at four embodiments above with severaladvantages. A traditional style discrete component optical pickup headfor disk drives is shown in FIG. 1 and FIG. 2. Every component, laser,lens, beamsplitter, mirror, etc., are discrete and separately mounted ina large housing. Another pickup design sometimes known as a holographichybrid pickup is shown as prior art in FIG. 3. This conventional designuses a diffractive or holographic element to serve the dual functions ofbeamsplitter and servo-generating element. However, it still requiresseparately assembly of many of the components within the holographicmodule and must be mounted in a pickup housing with a separate objectivelens giving an overall size and weight comparable to a traditionaldiscrete component pickup design. The present invention gives a completepickup with the same functionality as the traditional discrete componentdesign or holographic hybrid design, but with a small fraction of thesize and weight. Due to this advantage, the present pickup enables thedesign of disk drives of smaller physical size or the use of multiplepickups within a single drive, for example, to read multiple disksurfaces. Moreover, unlike the traditional or holographic hybridpickups, which require separate assembly of each pickup, the presentinvention is conducive to mass production of many pickups in parallel.The primary optical and optoelectronic elements are all produced usinglithography and standard batch processing procedures, so that manypickups are made at once rather than one at a time. This willsignificantly reduce the cost of producing each pickup.

[0045] A prior art pickup design using two mirrored flat surfaces and asingle DOE surface has been disclosed in the U.S. Pat. No. 5,621,716. InU.S. Pat. No. 5,621,716, the optical path length is reduced by foldingit using reflective surfaces. Another prior-art pickup design using twotransmissive DOE surfaces has been disclosed in the U.S. Pat. No.5,805,556. The first DOE surface, nearest to the laser, serves the dualfunctions of expanding the divergence of the laser beam so as to shortenthe optical path length and providing beamsplitter/servo-generatingfunctions for the return beam after reflection from the disk. The secondDOE surface focuses the expanding beam at high NA onto the disk.However, these two prior-art pickup designs above are still not at anefficient design. The present invention provides new pickup designs withmany important advantages over the prior-art designs.

[0046] In conclusion, the present invention includes several advantagesas follows:

[0047] 1. The invention has at least an advantage of diffractivereflective surfaces over planar reflective surfaces. Embodiment 1 of theinvention uses diffractive reflective surfaces to expand the beamdivergence while folding the optical path. In this way, the optical pathlength and the size of the pickup can be made appreciably smaller thanthe prior-art design because the prior art provides no means forexpanding the beam divergence.

[0048] 2. The invention has at least an advantage of sharing themultiple optical functions between multiple diffractive surfaces. Theprior art has only a single diffractive surface to provide all of theoptical functions. The optical functions include focusing the light beamat high NA onto the disk, and serving as thebeamsplitter/servo-generating element, transforming the beam returningfrom the disk such that its intensity pattern on the signalphotodetectors allows the FES, TES and RF information signal to beextracted. There are many advantages to the approach used in the presentdesign where a stack of multiple diffractive surface shares the multipleoptical functions.

[0049] 3. The invention has at least an advantage of maintaining acomparatively large minimum groove spacing. For a reasonable lens NA,such as NA=0.6 used in digital versatile disk (DVD) system, the minimumgroove spacing required for focusing the light with a single diffractivesurface is less than a micron. The diffraction efficiency is unavoidablyand dramatically reduced when the groove spacing for a lens becomes soclose to the wavelength of light. Thus the effective profile of the beamthat is being focused is distorted due to high efficiency at the centerof the lens where the groove spacing is large and low efficiency at theperiphery of the lens where the groove spacing is small. Ultimately,this produces a poorly focused spot on the disk. With a stack ofdiffractive surfaces, the minimum groove spacing for each surface can bekept large enough to avoid the distortion due to non-uniform diffractionefficiency.

[0050] 4. The invention has at least an advantage of using a separateDOE surface for the beamsplitter/servo-generating function. There isalso a significant advantage in using a separate DOE surface differentfrom the diffractive lens surfaces, as in the present invention, toimplement the beamsplitter and servo-generating element functions.Combining all of the optical functions into the same diffractive surfaceas in the prior art both necessitates sharing the diffraction efficiencybetween the lens and beamsplitter/servo-generating functions andunavoidably creates spurious diffraction orders arising from crosstalkbetween the two dissimilar optical functions. Even with idealperformance, the best efficiency that can be achieved would be less than25%. (This is estimated by assuming 50% of the light into the lensfocusing function, 50% of the light into thebeamsplitter/servo-generating function. Also and, only half the lightgoes into the focused spot on the disk and only half of that is steeredby the beamsplitter onto the photodetectors.) In fact, the power intothe spurious diffraction orders due to crosstalk will also besignificant so that the efficiency will be even less than this idealvalues. Furthermore, the spurious diffraction orders give rise tosignificant stray light in the system that may degrade the pickupperformance. All of these problems are avoided by using a separatepolarization selective DOE surface for the beamsplitter/servo-generatingfunction. Due to its polarization-selective nature, there is nodiffraction by this surface as light propagates towards the disk. Thusthe diffraction efficiency can be maintained at near 100% for both thelight being focused onto the disk and the return light steered onto thephotodetectors. Furthermore, since the invention implements the lens andthe beamsplitter/servo-generating optical functions using independentDOE surfaces, there will be no crosstalk between these functions, andtherefore no spurious diffraction orders to waste light and createstray-light noise problems.

[0051] 5. The invention has at least an advantage of using multiple DOEsurfaces to compensate chromatic aberration effects. Diffractivesurfaces are known to be very dispersive. Diffraction is a strongfunction of wavelength. In the operation of a pickup head, wavelengthshifts of the laser due to aging and changing temperature areunavoidable. This may degrade the performance of the diffractiveelements. By using multiple DOE surfaces to implement the diffractivelens as in the invention, positive and negative focusing powers can bebalanced between the multiple surfaces so as to provide the desireddegree of achromaticity. This is impossible if only a single diffractivesurface is used as disclosed in the prior art design.

[0052] It will be apparent to those skilled in the art that variousmodifications and variations can be made to the structure of the presentinvention without departing from the scope or spirit of the invention.In view of the foregoing, it is intended that the present inventioncover modifications and variations of this invention provided they fallwithin the scope of the following claims and their equivalents.

What is claimed is:
 1. A pickup head suitable for uses in an opticaldata storage system, the pickup head comprising: a diffractive opticalelement (DOE) stack on a semiconductor substrate, wherein thesemiconductor substrate comprises a light source to emit a light beamtoward an optical storage medium of the optical data storage system, anda plurality of photodetectors for receiving the light beam reflectedfrom the optical storage medium, wherein the DOE stack comprises: aplurality of diffractive lens surfaces located on a top side and abottom side of the DOE stack, wherein the bottom side is located on thesemiconductor substrate, and the top side is directed to the opticalstorage medium of the optical data storage system; and a middle DOElayer sandwiched between the diffractive lenses, comprising abeamsplitter/servo-generating DOE for serving functions of beamsplitterand servo-generating for the light beam reflected from the opticalstorage medium of the optical data storage system.
 2. The pickup head ofclaim 1, wherein the middle DOE layer comprises a polarization-selectiveDOE and a quarter-wave retarder, in which the quarter-wave retarder canchange the polarization direction of a passing light, so that the lightbream reflected from the optical storage medium can be diffracted by thepolarization-selective DOE.
 3. The pickup head of claim 1, wherein thebeamsplitter/servo-generating DOE comprises four quadrant portions, eachof which diffracts the light beam reflected from the optical storagemedium onto one corresponding sub-photodetector of each of thephotodetectors, respectively, so as to produce a focus error signal(FES), a track error signal (TES), and an information signal.
 4. Thepickup head of claim 1, wherein the optical storage medium comprises anoptical disk.
 5. The pickup head of claim 1, wherein the diffractivelenses on the top side of the DOE stack provides two lens surfacesserving as two transmissive DOE surfaces, and the diffractive lenses onthe bottom side of the DOE stack provide an upper DOE surface and alower DOE surface, wherein the upper DOE surface has a contact with themiddle DOE layer and performs as a combined reflective and transmissivesurface, and the lower DOE surface has a contact with the semiconductorsubstrate and perform as a reflective surface, which also has openingsto let the light beam from the light source and onto the photodetectorspass through, whereby as the light beam from the light source enters thelower DOE surface, an optical path length of the light beam is foldedbetween the lower DOE surface and the upper surface, and the light beamis focused onto the optical storage medium through the two transmissiveDOE surfaces, and as the light beam reflected from the optical storagemedium is deflected onto the photodetectors, the optical path can beoptionally folded between the lower DOE surface and the upper DOEsurface depending on a diffraction angle on the upper DOE surfacerelative to locations of the photodetectors.
 6. The pickup head of claim5, wherein the light source comprises an edge emitting laser diode witha turning mirror/beamshaper on the semiconductor substrate so that thelight beam emitted from the light source can enter the lower DOE surfacethrough the opening.
 7. The pickup head of claim 5, wherein the lightsource comprises a vertical cavity surface emitting laser (VCSEL) toemit the light beam directly toward to the optical storage mediumthrough the opening without a turning means to preliminarily turn thelight beam from the VCSEL.
 8. The pickup head of claim 1, wherein thediffractive lenses on the bottom side of the DOE stack provides a firsttransmissive DOE surface on the semiconductor substrate, and thediffractive lenses on the top side of the DOE stack provides a secondtransmissive DOE surface and a third transmissive DOE surface, whereinthe first transmissive DOE surface expands divergence of the light beam,the second and third transmissive DOE surfaces are used together toachieve a desired focusing power with reasonable minimum designconditions, whereby the light beam from the light source can be expandedand focused onto the optical storage medium through the first, second,and third transmissive DOE surfaces, and the light reflected from theoptical storage medium can be focused and steered onto thephotodetectors by the combined action of the first, second and third DOEsurfaces and the beamsplitter/servo-generating DOE sandwiched in themiddle of the DOE stack
 9. The pickup head of claim 8, wherein the lightsource comprises an edge emitting laser diode with a turningmirror/beamshaper on the semiconductor substrate so that the light beamemitted from the light source can enter the first transmissive DOEsurface.
 10. The pickup head of claim 9, wherein the light sourcecomprises a vertical cavity surface emitting laser (VCSEL) to emit thelight beam directly toward to the optical storage medium without aturning means to preliminarily turn the light beam from the VCSEL.
 11. Amethod for fabricating a stacked optical pickup head, the methodcomprising: providing a substrate, which comprises a plurality ofphoto-detecting systems arranged in a cell array, and each of thephoto-detecting systems comprises a light source and a multi-elementphotodetector formed thereon, wherein the light source and themulti-element photodetector are used in the stacked optical pickup headto allow accessing data stored in an optical data storage medium at adesired position; performing a semiconductor-like fabrication process toform a preliminary diffractive-stack-element (DOE) stack, which comprisea plurality of functional layers used for performing a desired functionof a DOE stack, wherein the preliminary DOE stack comprises a pluralityof DOE cells arranged in an regular array associating in space with thecell array of the photo-detecting systems; slicing the array of the DOEcells into a plurality of I-dimensional arrays; precisely locating the1-dimesional arrays on the substrate so that the DOE cells are alignedto the cells of the cell array of the photo-detecting systems; dicingthe substrate and the preliminary DOE stack into a plurality ofindividual stacked pickup heads with respect to the DOE cells.
 12. Themethod of claim 11, wherein the functional layers of the preliminary DOEstack are fabricated by steps comprising: forming at least one lowerdiffractive lens layer, which contacts with the substrate and providesat least one diffractive DOE surface; forming abeamsplitter/servo-generating DOE layer on the at least one lowerdiffractive lens layer, which optionally may or may not be apolarization-selective DOE layer; forming a quarter-wave retarder layerif the previous beamsplitter/servo-generating DOE layer is apolarization-selective DOE layer to rotate a polarization direction ofpassing light, wherein the polarization-selective DOE layer and thequarter-wave retarder layer are used together to diffract a portion of alight beam reflected from the storage medium onto the multi-elementphotodetector; and forming at least one upper diffractive lens layer onthe quarter-wave retarder layer, used to provide at least onediffractive DOE surface, wherein the diffractive DOE surfaces of thelower and the upper diffractive lens layers are together used to expandlight convergence and to achieve a desired focusing power.
 13. Themethod of claim 12, wherein patterns of the DOE cells of the functionallayers of the preliminary DOE stack are formed by processes comprisingphotolithography and etching.
 14. The method of claim 11, wherein thesubstrate comprises a semiconductor substrate.
 15. The method of claim11, wherein the light source on the substrate comprises a laser source.16. The method of claim 15, wherein the light source comprises an edgeemitting laser diode associating with a turning mirror/beamshaper formedthereon also for turning a light direction from the laser diode to theDOE stack.
 17. The method of claim 15, wherein the light sourcecomprises a vertical cavity surface emitting laser (VCSEL).